5-Thio-D-glucose, the nearest analog of D-glucose, inhibits active cellular transport of D-glucose. 5-Thio-D-glucose, while phosphorylated by hexokinase, acts as a mild competitive inhibitor to D-glucose phosphorylation but 5-thio-D-glucose 1-phosphate, thought a substrate for phosphoglucomutase, is a strong competitive inhibitor. Hence, the thiosugar or its glycolytic products may greatly inhibit normal glycolysis and thus exhibit a second inhibition to D-glucose utilization by mammalian cells. These effects will be examined. Since thiosugar is slowly metabolized in mammals to CO2, the glycolytic reactions are of interest at least to desulfurization and formation of normal components of metabolism. These justifiable reaction sequences and interactions will be examined using standard enzyme kinetics, isolation procedures and product identification methods. 5-Thio-D-fructose and 6-thio-D- fructose will be synthesized for use as enzyme substrates and inhibitors.